To diagnose potential problems with a communication line within a telecommunication network, several measurements techniques have been utilized. At the most basic level, the metallic loop test (MLT) is a measurement technique that has been used to test a direct current (dc) component of the connection between the subscriber and an end-office, commonly known as the loop connection. MLT determines whether a short circuit exists in the communication line that would prevent transmission, but cannot evaluate the transmission quality of an operating communication line.
To assess the transmission quality of a communication line, other measurement techniques have been developed. For example, one technique is to take a segment of the communication network, for example the communication path between two toll switches, off-line/out-of-service and then perform measurements on that particular segment. This process is then repeated for other segments of the communication network to isolate the origin of the problem. Such a measurement technique, however, presents several problems. First, substantial costs result from taking segments of the communication network off-line to perform testing. Second, every segment of the communication network may not be owned by the same commercial entity. In such a case, performing and coordinating a test of the communication path from the call origin to the call destination may not be feasible. Third, each time a communication line is established within a telecommunication network, the specific path of the communication line likely employs different transmission facilities/trunks. Therefore, the problem may appear intermittently on calls between two locations and may require extensive testing to identify the source of the problems. Furthermore, this measurement technique can only be performed on the limited portion of the communication line between the two end-office switches which is generally not the cause of transmission problems. Therefore, the portion of the communication line that is frequently the source of transmission problems, the portion of the communication line between the end-office switch and the subscriber's telephone set, cannot be tested using this measurement technique.
Measurement techniques utilizing a device known as a transmission "test line" have been developed to evaluate portions of the communication line. The test line device is located at a switch. Typically, a maintenance technician would be sent to the subscriber's premises to remove the subscriber's telephone set and then to install the measurement device, which would make a connection to the test line. Measurements are then performed through the transmission of various test signals between the near-end (i.e., maintenance facility's end) test equipment and the far-end test line device. For example, the 100 test line is an industry-wide device used to test loss and noise on a connection.
The use of test line equipment, however, suffers several shortcomings. First, this testing is entirely intrusive and cannot be performed while the subscriber is on the line. Thus, the subscriber cannot provide the maintenance technician any simultaneous feedback during the course of measurements. Second, this testing requires sending a technician to the customer's location which is expensive.
Similar to test line equipment, U.S. Pat. No. 5,490,199 issued to Fuller et al., on Feb. 6, 1996, and assigned to AT&T Corp. discloses a system for monitoring and analyzing facsimile transmissions. Measurements are performed non-intrusively and in real time during the transmissions of facsimiles and thus require a facsimile machine connected at both ends of the connection. These measurements, however, cannot be performed on a connection between any devices other than facsimile machines. Thus, just like test line equipment, these measurements require the subscriber's telephone set to be removed from the communication line. Furthermore, because these measurements cannot be performed while the subscriber is talking on the line, the subscriber cannot provide the maintenance technician any simultaneous feedback.